This invention relates to silicone rubber curing compositions in which bromobenzoyl peroxides are used as improved curing catalysts.
Cured or vulcanized silicone elastomers have become valuable materials owing to the attractive low temperature (-150.degree. F.) and high temperature (600.degree. F.) physical properties, low compression set, physical strength, vibration damping ability, resistance to weathering, chemical resistance, biological inertness, fungus resistance, electrical properties, and release properties. Because of these attractive properties vulcanized silicone rubber parts and articles have found increasing use in high temperature under-the-hood automotive applications (e.g., insulation for ignition wiring, heater hoses, gaskets, "O" rings, rubber boots, etc.), aerospace applications (e.g., jet and rocket engine parts, ablative coatings, etc.), food and health applications (e.g., conveyor belts for handling of sticky foods, nipples for baby bottles, prosthetic devices, and surgical tubing), high voltage electrical applications (e.g., insulation for wires at power plants) and construction applications (e.g., glazes).
In order to fabricate such useful parts and articles processable silicone rubber compounds containing an organopolysiloxane gum, reinforcing filler(s), additives and a curing (vulcanizing) agent must be formed, molded, or extruded and subjected to elevated temperatures (115.degree.-175.degree. C.) in order to shape and crosslink the silicone rubber composition. A post-cure and possibly also a heat aging are often needed to drive off decomposition products derived from the curing agent and other volatile materials that may be present. These treatments result in improved compression set and electrical properties and reduced odor and outgassing.
The most attractive curing agents for silicone rubber compounds are those that are very active and vulcanize at lower temperatures. Hence, energy requirements for curing are lower and cured silicone rubber parts or articles can be made faster and at lower unit costs. The most active peroxide curing agents are aromatic diacyl peroxides such as benzoyl peroxide (BPO) 1, di-(2,4-dichlorobenzoyl)peroxide 2, di-(2-chlorobenzoyl)peroxide 3, and di-(4-chlorobenzoyl)peroxide 4. Up to the present time the most active silicone rubber curing agents have been 2 and 3. 4 and BPO are the slowest of the active curing catalysts for silicone rubber compounds. Although these active curing catalysts are effective curing agents for silicone rubber compounds, there are some disadvantages to using them as curing agents for silicone rubbers. The use of 2 for curing of silicone rubber compounds results in vulcanized articles that exhibit undesirable blooming effects (British Pat. No. 1,595,534). In addition, it is generally known by peroxide producers, silicone rubber producers and fabricators, and the US EPA that chlorobenzoyl peroxides such as 2, 3, and 4 decompose thermally to form significant amounts of toxic polychlorinated biphenyls (PCB's) (Federal Register, Vol. 48, No. 237, 12/8/83, PP. 55076-98, EPA Proposed Rules, 40 CFR Part 761, Polychlorinated Biphenyls (PCB's); Exclusions, Exemptions and Use Authorizations). Hence, silicone rubber articles vulcanized with 2, 3 and 4 are likely to contain detectable amounts of PCB's such as 2,2',4,4'-tetrachlorobiphenyl, 2,2'-dichlorobiphenyl and 4,4'-dichlorobiphenyl, respectively. This is highly undesirable, especially when the vulcanized articles are used in contact with food (such as food conveyor belts) or are used in prosthetic devices. The threat of PCB's not only affects the fabricators and end-users of silicone rubber articles but it also deleteriously affects the producers of chlorobenzoyl peroxides. If these chlorobenzoyl peroxides are handled significantly above their recommended storage temperatures, thermal decompositions could occur, thus exposing workers and the environment to toxic PCB's. When these incidents occur, costly clean-ups and disposals and time consuming reporting to the US EPA and state and local agencies are required.
Thus, there is a need for an active silicone rubber curing catalyst which has activity similar to that of 2 and which does not decompose thermally to form undesirable and toxic PCB's. It has now have surprisingly and unexpectedly found that di-(2-bromobenzoyl)peroxide 5, a silicone rubber curing agent used in this invention, satisfies both of these criteria. Decomposition of this peroxide does not result in formation of toxic PCB's. Furthermore, 5 was found to be significantly more efficient in curing of a silicone rubber composition than was 2 when both were evaluated at equivalent levels in the silicone rubber composition.